I am struggling with a similar issue. '06 F150, XLT. During the summer air would blow perfectly out of whichever vents were selected (dash, floor, defrost). After driving for awhile I noticed amount of air coming out vents would reduce but could hear fan still at full speed and sound of air coming out behind the dash. Was not really a problem until now when windows are iced in the morning. At operating temp am only getting luck warm air and blower still sounds like its blowing behind glovebox. I have changed both blend door actuators (temp and mode). Only other thing I can come up with is jammed/broken temp door. Does anyone have a good reference on how to access this door without cutting the plenum open?